FORMATION OF SPINEL IN MELT OF THE MgO—Al2O3—S i O 2—C a F 2 SYSTEM AGGLOMERATED WELDING FLUX AND ITS EFFECT ON VISCOSITY OF SLAG I.A. GONCHAROV1, V.E. SOKOLSKY2, A.O. DAVIDENKO2, V.I. GALINICH1 and D.D. MISHCHENKO1 1E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine 2Taras Shevchenko National University, Kiev, Ukraine X-ray examinations of structure of agglomerated flux of the MgO—Al2O3—SiO2—CaF2 system in solid and molten states evidence that in a temperature range above 1200 °C the Al2MgO4 hard spinel phase with a melting temperature of 2105 °C forms in the slag melt. It determines physical-chemical properties of the melt and, in particular, the smooth character of viscosity changes in a temperature range of 1180—1540 °C. By manipulating proportions and concentrations of spinel-forming components Al2O3 and MgO, it is possible to achieve the optimal values of the temperature dependence of viscosity of the slag melt and, on this base, develop welding fluxes with predictable technological properties. Keywords: welding, agglomerated flux, structure properties in welding of high-strength steels, nor- of slag melts, viscosity, diffraction examinations, spinel mally the use is made of high-basicity fluxes with Conventional-strength steels have been gradu- an increased content of calcium oxide and fluo- ally replaced lately by increased- and high- ride. However, they fail to provide stability of strength steels. To weld such steels it is necessary the welding process and quality formation of the to use a number of appropriate metallurgical welds in multi-arc welding of pipes at a speed of processes and consumables. more than 100 m/h, as they are «short», unlike Welding of conventional-strength steels is the manganese-silicate fluxes. Agglomerated fluxes are finding growing ap- performed by using the MnO—SiO —CaF slag 2 2 plication in welding of high- and increased- system fluxes characterised by good welding- strength steels. This is attributable to their wider technological properties. The latter are achieved metallurgical possibilities of affecting the weld due to the fact that structure of the melts of these pool metal. In general, the problems of sub- fluxes is determined by a silicate skeleton of a merged-arc welding of low-alloy high-strength differing polymerisation degree, which, in turn, steels by using agglomerated fluxes are consid- determines physical-chemical properties of slag ered in detail in studies by I.K. Pokhodnya [2], in the molten state at temperatures of existence D. Olson [3], T. Eagar [4], etc. At the same of the weld pool. To provide formation of the time, there are almost no studies dedicated to defect-free welds in submerged-arc welding the the fundamental research of structure, physical- slag pool should damp oscillations of the metal chemical properties of the slag melts in connec- pool. Therefore, slags must be characterised by tion with technological properties of the welding a smooth increase in viscosity with decrease in fluxes. temperature, i.e. they must be «long» [1]. How- There are several theories describing structure ever, despite good technological properties, the of the welding slag melts. But all of them apply manganese-silicate fluxes have limited possibili- to the equilibrium processes of metallurgy [5—8] ties in terms of affecting the values of mechanical characterised by complete remelting of slag com- properties of the weld metal, and impact tough- ponents and formation of the liquid slag phase. ness in particular. This is explained by the fact Unlike the metallurgical processes, the welding that welding by using these fluxes involves the processes are rapid. They feature substantial tem- processes of formation of silicate non-metallic perature gradients in different zones of the weld inclusions and reduction of silicon in the weld pool and, hence, are far from being equilibrium. metal, which are undesirable for increased- Moreover, in melting during the welding process strength steels. To achieve required mechanical the majority of the agglomerated fluxes used now © I.A. GONCHAROV, V.E. SOKOLSKY, A.O. DAVIDENKO, V.I. GALINICH and D.D. MISHCHENKO, 2012 18 12/2012 are not a homogeneous liquid, like the fused The E.O. Paton Electric Welding Institute fluxes, but a heterogeneous mixture of different studied the temperature dependence of viscosity phases, some of which are crystalline. At present, of the investigated flux and a number of model there are no slag theories describing the liquid- fluxes of the same slag system in order to deter- crystalline structure of a slag, which does not mine the effect of the solid phase in a liquid slag allow theoretical estimation of the effect of the melt on physical-chemical properties. Viscosity crystalline component of the slag on the processes of slags was measured by using the rotational occurring in the weld pool. viscometer in the Tamman furnace in a purified Coarse crystallites present in the matrix of a argon flow. liquid part of the slag must have a considerable Scanning electron microscope JSM-7700F effect on viscosity and, hence, welding-techno- with the X-ray spectral microanalyser was used logical properties of the flux. Therefore, one of for obtaining images of the initial and remelted the purposes of this study was to qualitatively fluxes, and for local chemical analysis of microin- reveal the effect of the crystalline inclusions on clusions. viscosity of the slag. The powdered specimen of the flux on a graph- For a number of years we have been develop- ite substrate and the solidified specimen of the ing agglomerated welding fluxes based on the slag produced from it by remelting in the molyb- MgO—Al2O3—SiO2—CaF2 slag system. The re- denum crucible at a temperature of 1500 °C in sults obtained prove the possibility of developing the high-purity helium atmosphere were sub- a new generation of welding fluxes based on the jected to electron optical examinations. The slag above system. Study [9] describes investigations specimen was examined on the side of both bot- of structure of a welding flux of the given system tom and surface of the molybdenum crucible. To with a low MgO content (10 wt.%), and study prevent the effect of charging of the specimen by [10] – investigations of the processes of melting the electron beam, a 3 nm thick layer of pure and solidification of fluxes of the above slag sys- platinum was sprayed on the specimen surface. tem. As established in study [11], increase in the Filming of the flux specimen was carried out content of magnesium oxide in fluxes of the at temperatures of 600, 800, 1000, 1200, 1300, MgO—Al2O3—SiO2—CaF2 system leads to de- 1400 and 1500 °C in a high-temperature vacuum crease in thermodynamic activity of SiO2 in the chamber in the high-purity helium atmosphere. slag melt. This fact is very important in terms of Chemical composition of the specimens was metallurgy of welding of high-strength steels, as controlled by the X-ray fluorescent analysis it makes it possible to reasonably suppress the method. undesirable processes of silicon reduction and for- Structural software Powdercell and Mercury, mation of silicate non-metallic inclusions in the data bases Match and Retrieve, which are dis- weld metal. seminated free of charge through the Internet, Experimental conditions and analysis of ex- were used to interpret the X-ray analysis data. perimental data. X-ray examinations of structure The software developed in-house was used in cal- of molten and solid slags were carried out at the culations for investigation of the slag melt [12]. Physical Chemistry Chair of the Taras Shevchen- Investigations at room temperature. The cal- ko National University by using the upgraded culated contents of the main components of the X-ray diffractometer for examinations of melts. flux, i.e. MgO, Al2O3, SiO2 and CaF2, are given Monochromatisation of MoKα-radiation was per- in Table 1. Liquid glass Na—K was added to the formed by using a pair of balanced differential refined mechanical mixture in the granulator. Af- filters Zr—Y. Earlier, the diffactometer was used ter granulation, the flux was held for 24 h at a to advantage for examination of fused fluxes in temperature of 20 °C, and then was baked for glassy and solid states [12]. After upgrading, this 1 h at 500 °C. The actual chemical composition diffractometer allows the diffraction examina- of the flux, according to the X-ray fluorescent tions of specimens to be performed both in the analysis data, is also given in Table 1. liquid and solid states in a temperature range of Results of electron microscopic examinations 100—1700 °C. After granulation and drying of a of the powdered agglomerated flux indicate that flux, it was ground into a powder and subjected Table 1. Composition of welding flux, wt.% to X-ray analysis (CuKα-radiation, diffractome- ter DRON-3M). Also, investigations were car- Type of analysis MgO Al2O3 SiO2 CaF2 Na2OK2OFe2O3 ried out with a solid specimen of the slag after Calculated 30 25 20 25 — — — holding at 1500 °C on the sides of the crucible Actual 25.8 18.7 28.4 22.2 1.58 0.8 1.88 bottom and surface. 12/2012 19 bottom sides differ to some extent. The crystal- line phases formed on the bottom side are finer (Figure 2, spectrum 3). It can be clearly seen that the continuous matrix of one of the phases (the lightest one) contains inclusions of other, finer phases. It is practically impossible to iden- tify this phase as one compound. Most probably, this phase was formed as a result of melting of low-melting point components of the flux and dissolution of some solid components in the melt.